Accurate measurement of peripheral vision in a human can lead to early diagnosis of potentially serious medical conditions. Causes of visual field deficit include neonatal brain haemorrhage or stroke and eye conditions such as retinitis pigmentosa and glaucoma, additionally over 50% of childhood brain tumours present with visual impairment.
A human eye converts light entering it into neural signals which are then sent down the optic nerve. Thus, the left optic nerve carries all of the visual information from the left eye, and the right optic nerve carries all of the visual information from the right eye. The optic nerves meet at the optic chiasm within the brain. Within the optic chiasm nerve fibres carrying visual information from the visual field temporal (lateral) to the vertical mid-line, cross to the other side. The resultant right optic tract therefore consists of nerve fibres subserving the temporal visual field of the left eye and the nasal visual field of the right eye and the left optic tract consists of nerve fibres subserving the temporal visual field of the right eye and the nasal visual field of the left eye these are called homonymous visual hemi-fields. The optic tracts carry the left and right homonymous hemi-field information to the occipital lobe of the brain. The left homonymous visual hemi-field is processed by the right occipital cortex and the right homonymous visual hemi-field is processed by the left occipital cortex.
The pattern of visual field loss will therefore allow the clinician to determine the position of a tumour or brain injury. An ocular abnormality such as glaucoma or retinal detachment will cause a visual field defect which crosses over the vertical mid-line. A tumour on or touching the optic nerve will result in loss of the central visual field in one eye along with reduction of central visual acuity. Tumours involving the optic chiasm often do not initially affect central visual acuity but interrupt the nerve fibres subserving the temporal visual field as they cross within the chiasm—causing a temporal hemi-field loss in both eyes (bitemporal hemianopia). Bitemporal hemianopias do not usually cause symptoms of visual disturbance but are important to detect in order to diagnosis a chiasmal tumour at an early stage. Tumours or brain injury involving the optic tract and occipital cortex will cause a homonymous visual field defect—for example a tumour in the right occipital lobe will cause loss of the temporal hemi-visual field from the left eye and the nasal visual field in the right eye. This would be termed a left homonymous hemianopia.
Accurate measurement of peripheral vision can be a first indicator of brain tumours and, as described, the pattern of visual field loss can localise the tumour or other pathology. Serial measurements of the peripheral visual field defect can aid in the monitoring of tumour growth and indicate if further therapy is required.
Several techniques are known for measuring peripheral vision of a human patient. Two common perimetry measurement apparatuses are the “Goldman perimeter”, and the “Humphrey field analyser”. However, both sets of apparatus require the human patient to place their head into an enclosed compartment, and to maintain their vision at a fixation point in the centre of the compartment, suppressing their reflexive eye movement toward the appearing target in the visual periphery. Such techniques are therefore difficult to use with small children, for example, because small children find the test extremely intimidating, and are unable to maintain visual fixation at the central point in accordance with the test instructions appropriately. Similarly, elderly patients who may be suffering from degenerative brain diseases also present the same problems.
In order to get around such problems various other techniques have been developed to try and measure peripheral vision in small children. FIG. 1 illustrates a diagram from Suga et al, “Development of a Quantative Perimeter Screening System for Young Children Based on an Immersive VR Display”, Electronics and Communications in Japan, Part II, Volume 89, No. 11, 2006. Here, a virtual reality technique is used to construct a pseudo video space. The immersive display device has three screens measuring 3 m×2.25 m on the front and the two sides, and also a 3 m×1 m vertical screen at the bottom. The system includes four projectors to produce video images on the respective screens, four computers to send the video signals to the projectors, and a computer to provide synchronisation signals to these computers.
In order to provide a fixation point for the child subject, a video image which attracts the attention of the subject is used. Then, while the video image is being displayed at the fixation point, image targets, which are simply round circles of light, are made to appear on peripheral screens. A single camera is provided focused on the subject's face, and image processing is used to determine the view line of the subject i.e. the direction of gaze of the subject, using template matching. At the time a target is displayed, if, as a result of the image processing it is determined that the subject view line changes within one second of the target presentation in a direction agreeing with the direction of the target presentation, it is judged that the target has been recognised.
The Suga et al system therefore presents an attempt at producing an automated perimeter measurement system, using image processing of the subject to try and determine the subject's direction of gaze, and whether a target has been detected. However, the system is extremely complicated, requiring much space, and equipment to set up. In addition, the actual criterion by which it is judged whether a target has in fact been seen is open to error, as a decision is made purely based on whether the subject looks towards the direction of the target, but not whether in fact the target has been seen.
Other, manual, techniques are also known. FIG. 2 illustrates the “white sphere kinetic arc” method. Using a white ball as a fixation point, another white ball is moved into a child's visual field by a clinician, and the child's response is monitored by a second clinician. This method requires two clinicians, and the movement of the second clinician may be distracting, leading to an inaccurate test. As such, this method is far from ideal, delivering a potentially poor result, as well as being costly to administer.
There is thus a need for a visual perimeter measurement system that can be used with small children, and that will provide more reliable, accurate, and consistent results.